Microlenses for high dynamic range imaging pixels

ABSTRACT

An image sensor may include high dynamic range imaging pixels having an inner sub-pixel surrounded by an outer sub-pixel. To steer light away from the inner sub-pixel and towards the outer sub-pixel, the high dynamic range imaging pixels may be covered by a toroidal microlens. To mitigate cross-talk caused by high-angled incident light, various microlens arrangements may be used. A toroidal microlens may have planar portions on its outer perimeter. A toroidal microlens may be covered by four additional microlenses, each additional microlens positioned in a respective corner of the pixel. Each pixel may be covered by four microlenses in a 2×2 arrangement, with an opening formed by the space between the four microlenses overlapping the inner sub-pixel.

BACKGROUND

This relates generally to imaging systems and, more particularly, toimaging systems with high dynamic range functionalities.

Modern electronic devices such as cellular telephones, cameras, andcomputers often use digital image sensors. Imager sensors (sometimesreferred to as imagers) may be formed from a two-dimensional array ofimage sensing pixels. Each pixel receives incident photons (light) andconverts the photons into electrical signals. Each pixel is covered by acorresponding microlens. Image sensors are sometimes designed to provideimages to electronic devices using a Joint Photographic Experts Group(JPEG) format.

Conventional imaging systems also may have images with artifactsassociated with low dynamic range. Scenes with bright and dark portionsmay produce artifacts in conventional image sensors, as portions of thelow dynamic range images may be over exposed or under exposed. Multiplelow dynamic range images may be combined into a single high dynamicrange image, but this typically introduces artifacts, especially indynamic scenes.

It would therefore be desirable to be able to provide improved imagingsystems with high dynamic range functionalities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an illustrative electronic device withan image sensor that may include high dynamic range pixels in accordancewith an embodiment.

FIG. 2A is a cross-sectional side view of illustrative phase detectionpixels having photosensitive regions with different and asymmetricangular responses in accordance with an embodiment.

FIGS. 2B and 2C are cross-sectional views of the phase detection pixelsof FIG. 2A in accordance with an embodiment.

FIG. 3 is a diagram of illustrative signal outputs of photosensitiveregions of depth sensing pixels for incident light striking the depthsensing pixels at varying angles of incidence in accordance with anembodiment.

FIG. 4 is a top view of an illustrative pixel with an inner sub-pixeland an outer sub-pixel that is covered by a toroidal microlens inaccordance with an embodiment.

FIG. 5 is a top view of an illustrative pixel with an inner sub-pixeland a split outer sub-pixel group that is covered by a toroidalmicrolens in accordance with an embodiment.

FIG. 6 is a top view of illustrative toroidal microlenses that may beused to cover pixels of the type shown in FIGS. 4 and 5 in accordancewith an embodiment.

FIG. 7 is a top view of illustrative toroidal microlenses with mergedportions that may be used to cover pixels of the type shown in FIGS. 4and 5 in accordance with an embodiment.

FIGS. 8 and 9 are top views of illustrative toroidal microlenses havingouter perimeters with planar portions that may be used to cover pixelsof the type shown in FIGS. 4 and 5 in accordance with an embodiment.

FIG. 10 is a top view of illustrative microlens groups that include atoroidal microlens covered by four additional microlenses and that maybe used to cover pixels of the type shown in FIGS. 4 and 5 in accordancewith an embodiment.

FIG. 11 is a top view of illustrative microlens groups that include fourmicrolenses and that may be used to cover pixels of the type shown inFIGS. 4 and 5 in accordance with an embodiment.

DETAILED DESCRIPTION

Embodiments of the present invention relate to image sensors with pixelshaving high dynamic range functionality. These types of pixels mayinclude two or more photosensitive areas for collecting light. Toroidalmicrolenses or other microlens groups may be used to direct light to thetwo or more photosensitive areas. The toroidal microlenses may be shapedto direct light to the corners of the pixel to mitigate cross-talk athigh incident light angles. At high incident angles, the microlensshapes and arrangements discussed herein focus the light away from theboundary between adjacent pixels, thus mitigating the high angle lightfrom crossing over into the adjacent pixel. The pixels covered bytoroidal microlenses may also have phase detection functionality.

An electronic device with a digital camera module is shown in FIG. 1.Electronic device 10 may be a digital camera, a computer, a cellulartelephone, a medical device, or other electronic device. Camera module12 (sometimes referred to as an imaging device) may include image sensor14 and one or more lenses 28. During operation, lenses 28 (sometimesreferred to as optics 28) focus light onto image sensor 14. Image sensor14 includes photosensitive elements (e.g., pixels) that convert thelight into digital data. Image sensors may have any number of pixels(e.g., hundreds, thousands, millions, or more). A typical image sensormay, for example, have millions of pixels (e.g., megapixels). Asexamples, image sensor 14 may include bias circuitry (e.g., sourcefollower load circuits), sample and hold circuitry, correlated doublesampling (CDS) circuitry, amplifier circuitry, analog-to-digital (ADC)converter circuitry, data output circuitry, memory (e.g., buffercircuitry), address circuitry, etc.

Still and video image data from image sensor 14 may be provided to imageprocessing and data formatting circuitry 16 via path 26. Imageprocessing and data formatting circuitry 16 may be used to perform imageprocessing functions such as automatic focusing functions, depthsensing, data formatting, adjusting white balance and exposure,implementing video image stabilization, face detection, etc. Forexample, during automatic focusing operations, image processing and dataformatting circuitry 16 may process data gathered by phase detectionpixels in image sensor 14 to determine the magnitude and direction oflens movement (e.g., movement of lens 28) needed to bring an object ofinterest into focus.

Image processing and data formatting circuitry 16 may also be used tocompress raw camera image files if desired (e.g., to Joint PhotographicExperts Group or JPEG format). In a typical arrangement, which issometimes referred to as a system on chip (SOC) arrangement, camerasensor 14 and image processing and data formatting circuitry 16 areimplemented on a common integrated circuit. The use of a singleintegrated circuit to implement camera sensor 14 and image processingand data formatting circuitry 16 can help to reduce costs. This is,however, merely illustrative. If desired, camera sensor 14 and imageprocessing and data formatting circuitry 16 may be implemented usingseparate integrated circuits. If desired, camera sensor 14 and imageprocessing circuitry 16 may be formed on separate semiconductorsubstrates. For example, camera sensor 14 and image processing circuitry16 may be formed on separate substrates that have been stacked.

Camera module 12 may convey acquired image data to host subsystems 20over path 18 (e.g., image processing and data formatting circuitry 16may convey image data to subsystems 20). Electronic device 10 typicallyprovides a user with numerous high-level functions. In a computer oradvanced cellular telephone, for example, a user may be provided withthe ability to run user applications. To implement these functions, hostsubsystem 20 of electronic device 10 may include storage and processingcircuitry 24 and input-output devices 22 such as keypads, input-outputports, joysticks, and displays. Storage and processing circuitry 24 mayinclude volatile and nonvolatile memory (e.g., random-access memory,flash memory, hard drives, solid state drives, etc.). Storage andprocessing circuitry 24 may also include microprocessors,microcontrollers, digital signal processors, application specificintegrated circuits, or other processing circuits. It may be desirableto provide image sensors with high dynamic range functionalities (e.g.,to use in low light and high light environments to compensate for highlight points of interest in low light environments and vice versa). Toprovide high dynamic range functionalities, image sensor 14 may includehigh dynamic range pixels.

It may be desirable to provide image sensors with depth sensingcapabilities (e.g., to use in automatic focusing applications, 3Dimaging applications such as machine vision applications, etc.). Toprovide depth sensing capabilities, image sensor 14 may include phasedetection pixel groups such as phase detection pixel group 100 shown inFIG. 2A. If desired, pixel groups that provide depth sensingcapabilities may also provide high dynamic range functionalities.

FIG. 2A is an illustrative cross-sectional view of pixel group 100. InFIG. 2A, phase detection pixel group 100 is a pixel pair. Pixel pair 100may include first and second pixels such Pixel 1 and Pixel 2. Pixel 1and Pixel 2 may include photosensitive regions such as photosensitiveregions 110 formed in a substrate such as silicon substrate 108. Forexample, Pixel 1 may include an associated photosensitive region such asphotodiode PD1, and Pixel 2 may include an associated photosensitiveregion such as photodiode PD2. A microlens may be formed overphotodiodes PD1 and PD2 and may be used to direct incident light towardsphotodiodes PD1 and PD2. The arrangement of FIG. 2A in which microlens102 covers two pixel regions may sometimes be referred to as a 2×1 or1×2 arrangement because there are two phase detection pixels arrangedconsecutively in a line. In an alternate embodiment, three phasedetection pixels may be arranged consecutively in a line in what maysometimes be referred to as a 1×3 or 3×1 arrangement. In otherembodiments, phase detection pixels may be grouped in a 2×2 or 2×4arrangement. In general, phase detection pixels may be arranged in anydesired manner.

Color filters such as color filter elements 104 may be interposedbetween microlens 102 and substrate 108. Color filter elements 104 mayfilter incident light by only allowing predetermined wavelengths to passthrough color filter elements 104 (e.g., color filter 104 may only betransparent to the wavelengths corresponding to a green color, a redcolor, a blue color, a yellow color, a cyan color, a magenta color,visible light, infrared light, etc.). Color filter 104 may be abroadband color filter. Examples of broadband color filters includeyellow color filters (e.g., yellow color filter material that passes redand green light) and clear color filters (e.g., transparent materialthat passes red, blue, and green light). In general, broadband filterelements may pass two or more colors of light. Photodiodes PD1 and PD2may serve to absorb incident light focused by microlens 102 and producepixel signals that correspond to the amount of incident light absorbed.

Photodiodes PD1 and PD2 may each cover approximately half of thesubstrate area under microlens 102 (as an example). By only coveringhalf of the substrate area, each photosensitive region may be providedwith an asymmetric angular response (e.g., photodiode PD1 may producedifferent image signals based on the angle at which incident lightreaches pixel pair 100). The angle at which incident light reaches pixelpair 100 relative to a normal axis 116 (i.e., the angle at whichincident light strikes microlens 102 relative to the optical axis 116 oflens 102) may be herein referred to as the incident angle or angle ofincidence.

An image sensor can be formed using front side illumination imagerarrangements (e.g., when circuitry such as metal interconnect circuitryis interposed between the microlens and photosensitive regions) orbackside illumination imager arrangements (e.g., when photosensitiveregions are interposed between the microlens and the metal interconnectcircuitry). The example of FIGS. 2A, 2B, and 2C in which pixels 1 and 2are backside illuminated image sensor pixels is merely illustrative. Ifdesired, pixels 1 and 2 may be front side illuminated image sensorpixels. Arrangements in which pixels are backside illuminated imagesensor pixels are sometimes described herein as an example.

In the example of FIG. 2B, incident light 113 may originate from theleft of normal axis 116 and may reach pixel pair 100 with an angle 114relative to normal axis 116. Angle 114 may be a negative angle ofincident light. Incident light 113 that reaches microlens 102 at anegative angle such as angle 114 may be focused towards photodiode PD2.In this scenario, photodiode PD2 may produce relatively high imagesignals, whereas photodiode PD1 may produce relatively low image signals(e.g., because incident light 113 is not focused towards photodiodePD1).

In the example of FIG. 2C, incident light 113 may originate from theright of normal axis 116 and reach pixel pair 100 with an angle 118relative to normal axis 116. Angle 118 may be a positive angle ofincident light. Incident light that reaches microlens 102 at a positiveangle such as angle 118 may be focused towards photodiode PD1 (e.g., thelight is not focused towards photodiode PD2). In this scenario,photodiode PD2 may produce an image signal output that is relativelylow, whereas photodiode PD1 may produce an image signal output that isrelatively high.

The positions of photodiodes PD1 and PD2 may sometimes be referred to asasymmetric or displaced positions because the center of eachphotosensitive area 110 is offset from (i.e., not aligned with) opticalaxis 116 of microlens 102. Due to the asymmetric formation of individualphotodiodes PD1 and PD2 in substrate 108, each photosensitive area 110may have an asymmetric angular response (e.g., the signal outputproduced by each photodiode 110 in response to incident light with agiven intensity may vary based on an angle of incidence). It should benoted that the example of FIGS. 2A-2C where the photodiodes are adjacentis merely illustrative. If desired, the photodiodes may not be adjacent(i.e., the photodiodes may be separated by one or more interveningphotodiodes). In the diagram of FIG. 3, an example of the image signaloutputs of photodiodes PD1 and PD2 of pixel pair 100 in response tovarying angles of incident light is shown.

Line 160 may represent the output image signal for photodiode PD2whereas line 162 may represent the output image signal for photodiodePD1. For negative angles of incidence, the output image signal forphotodiode PD2 may increase (e.g., because incident light is focusedonto photodiode PD2) and the output image signal for photodiode PD1 maydecrease (e.g., because incident light is focused away from photodiodePD1). For positive angles of incidence, the output image signal forphotodiode PD2 may be relatively small and the output image signal forphotodiode PD1 may be relatively large.

The size and location of photodiodes PD1 and PD2 of pixel pair 100 ofFIGS. 2A, 2B, and 2C are merely illustrative. If desired, the edges ofphotodiodes PD1 and PD2 may be located at the center of pixel pair 100or may be shifted slightly away from the center of pixel pair 100 in anydirection. If desired, photodiodes 110 may be decreased in size to coverless than half of the pixel area.

Output signals from pixel pairs such as pixel pair 100 may be used toadjust the optics (e.g., one or more lenses such as lenses 28 of FIG. 1)in image sensor 14 during automatic focusing operations. The directionand magnitude of lens movement needed to bring an object of interestinto focus may be determined based on the output signals from pixelpairs 100.

For example, by creating pairs of pixels that are sensitive to lightfrom one side of the lens or the other, a phase difference can bedetermined. This phase difference may be used to determine both how farand in which direction the image sensor optics should be adjusted tobring the object of interest into focus.

When an object is in focus, light from both sides of the image sensoroptics converges to create a focused image. When an object is out offocus, the images projected by two sides of the optics do not overlapbecause they are out of phase with one another. By creating pairs ofpixels where each pixel is sensitive to light from one side of the lensor the other, a phase difference can be determined. This phasedifference can be used to determine the direction and magnitude ofoptics movement needed to bring the images into phase and thereby focusthe object of interest. Pixel blocks that are used to determine phasedifference information such as pixel pair 100 are sometimes referred toherein as phase detection pixels or depth-sensing pixels.

A phase difference signal may be calculated by comparing the outputpixel signal of PD1 with that of PD2. For example, a phase differencesignal for pixel pair 100 may be determined by subtracting the pixelsignal output of PD1 from the pixel signal output of PD2 (e.g., bysubtracting line 162 from line 160). For an object at a distance that isless than the focused object distance, the phase difference signal maybe negative. For an object at a distance that is greater than thefocused object distance, the phase difference signal may be positive.This information may be used to automatically adjust the image sensoroptics to bring the object of interest into focus (e.g., by bringing thepixel signals into phase with one another).

As previously mentioned, the example in FIGS. 2A-2C where phasedetection pixel block 100 includes two adjacent pixels is merelyillustrative. In another illustrative embodiment, phase detection pixelblock 100 may include multiple adjacent pixels that are covered byvarying types of microlenses.

FIG. 4 is a top view of an illustrative pixel that may be included in animage sensor such as image sensor 14. As shown, pixel 200 has at leasttwo different light collecting areas (LCAs). Pixel 200 may includephotodiodes with associated pixel circuitry used to capture the samespectrum of light. As an example, the pixels 200 may be used to capturered, green, blue, cyan, magenta, yellow, near-infrared, infrared, or anyother spectrum of light. A single red, green, blue, cyan, magenta,yellow, near-infrared, infrared, or clear color filter may be formedover the pixel 200. In certain embodiments, the color filter formed overpixel 200 may have areas that pass colored light and areas that areclear (i.e., that pass visible or full-spectrum light outside thevisible spectrum).

Pixel 200 of FIG. 4 may include a first sub-pixel 202, which may bereferred to as the inner sub-pixel 202. Inner sub-pixel 202 may becompletely surrounded by a second sub-pixel 204, which may be referredto as the outer sub-pixel 204. Inner sub-pixel 202 and outer sub-pixel204 may correspond to n-type doped photodiode regions in a semiconductorsubstrate. There may be respective sub-pixel circuitry in the substratesuch as transfer gates, floating diffusion regions, and reset gates ofthe pixel 200 that is coupled to the photodiode regions in thesub-pixels 202 and 204. The semiconductor substrate (not shown) may be abulk p-type substrate made of silicon, or any other suitablesemiconductor material.

A photodiode in inner sub-pixel 202 may have a circular shape at thesurface. In other words, the light collecting area of inner sub-pixel202 is a circular region. At the surface, the inner sub-pixel 202 mayhave a diameter S1. As an example, the diameter S1 of a photodiode ininner sub-pixel 202 may be 1 micron, but may alternatively be any otherdimension without departing from the scope of the present embodiment.Outer sub-pixel 204 may have a square outer boundary and a circularinner boundary at the surface. The area enclosed by the square outerboundary and circular inner boundary of outer sub-pixel 204 shown inFIG. 4 may correspond to the light collecting area of outer sub-pixel204. As shown in FIG. 4, the length of one of the sides of outersub-pixel 204 is S2. As an example, S2 may be 3 microns, but mayalternatively be any other dimension without departing from the scope ofthe present embodiment. The length S2 is preferably greater than thelength S1. Outer sub-pixel 204 is illustrated in FIG. 4 as having asquare outer boundary but may alternatively have a rectangular outerboundary or an outer boundary of any other desired shape (e.g., a curvedouter boundary such as a circular outer boundary).

If desired an optional isolation region may be formed between innersub-pixel 202 and outer sub-pixel 204. The isolation region may separateindividual sub-pixels in a given pixel from one another, and may alsoseparate individual sub-pixels in different respective pixels from oneanother. The optional isolation region may be formed from differenttypes of isolation devices such as trench isolation structures, dopedsemiconductor regions, metallic barrier structures, or any othersuitable isolation device.

Because inner sub-pixel 202 is surrounded by outer sub-pixel 204, innersub-pixel 202 may sometimes be described as being nested within outersub-pixel 204. Pixel 200 may sometimes be referred to as a nested imagepixel. The inner sub-pixel group and the outer sub-pixel group in anested image pixel may have the same geometric optical centers. In otherwords, because the outer sub-pixel group surrounds the inner sub-pixelgroup symmetrically, the center of the surface of the inner sub-pixelgroup is the same as the center of the outer sub-pixel group thatsurrounds the inner sub-pixel group.

The inner sub-pixel 202 may have a lower sensitivity to incident lightand may be referred to as having a lower sensitivity light collectingarea compared to outer sub-pixel 204. The respective dopingconcentrations of inner sub-pixel 202 and outer sub-pixel 204 may bedifferent or they may be the same. As an example, the dopingconcentrations of photodiode regions in inner sub-pixel 202 may bemodified to reduce the sensitivity of inner sub-pixel 202 to light.However, for the sake of simplicity in explaining and highlighting theproperties of the pixel 200, it will be assumed that the sub-pixels 202and 204 have photodiodes with the same doping concentrations. The lowersensitivity to incident light of inner sub-pixel 202 compared to outersub-pixel 204 may be a result of the lower light collecting area ofinner sub-pixel 202 compared to the light collecting area of outersub-pixel 204.

Inner sub-pixel 202 may sometimes be referred to as inner photodiode 202or inner photosensitive area 202. Similarly, outer sub-pixel 204 maysometimes be referred to as outer photodiode 204 or outer photosensitivearea 204.

The ratio of the light sensitivity of the outer sub-pixel group to thelight sensitivity of the inner sub-pixel group may be at least 4 to 1,but could be 5 to 1, 10 to 1, any intermediate ratio, or any largerratio. In other words, the light sensitivity of the outer sub-pixelgroup may be at least four times greater than the light sensitivity ofthe inner sub-pixel group.

One or more microlenses may be formed over the pixel 200 of FIG. 4 todirect light toward the outer sub-pixel 204. The one or more microlensesmay be formed over the color filter formed over pixel 200. To directlight toward outer sub-pixel 204, the one or more microlenses may beformed over only outer sub-pixel 204. As shown in FIG. 4, microlens 206is a toroidal microlens that covers outer sub-pixel 204. The toroidalmicrolens has an opening that overlaps inner sub-pixel 202 such that themicrolens does not overlap inner sub-pixel 202. This enables light to bedirected towards the outer sub-pixel. In some embodiments however, theone or more microlenses that direct light toward outer sub-pixel 204 maypartially or completely overlap the light collecting area of sub-pixel202. Directing light toward outer sub-pixel 204 may further increase thesensitivity of the light collecting area of outer sub-pixel 204 relativeto the sensitivity of the light collecting area of inner sub-pixel 202.In some embodiments, inner sub-pixel 202 may optionally be covered by amicrolens that is formed separately from microlens 206.

Because a larger amount of light incident on pixel 200 is directed toouter sub-pixel 204 than to inner sub-pixel 202, inner sub-pixel 202 issaid to have a lower sensitivity light collecting area compared to outersub-pixel 204. The difference in sensitivity to light of inner sub-pixel202 and outer sub-pixel 204 enables pixel 200 to be used in high dynamicrange applications while using the same integration time for eachsub-pixel. If desired, the integration time for each sub-pixel may bedifferent to further increase the dynamic range of the pixel.

It may be desirable to provide phase detection capabilities in a pixelof the type shown in FIG. 4. FIG. 5 shows an illustrative imaging pixelwith high dynamic range functionality and phase detection capability. Asshown in FIG. 5, pixel 200 may include an inner sub-pixel 202.Additionally, pixel 200 may include two outer sub-pixels 204-1 and204-2. Sub-pixels 204-1 and 204-2 may sometimes collectively be referredto as outer sub-pixel group 204. By splitting the outer sub-pixel group204 into two separate outer sub-pixels, the outer sub-pixel group mayhave phase detection capabilities (e.g., sub-pixels 204-1 and 204-2 mayeach have an asymmetric response to incident light). In FIGS. 4 and 5,toroidal microlens 206 is shown as not overlapping inner sub-pixel 202.This example is merely illustrative. If desired, toroidal microlens 206may partially overlap inner sub-pixel 202. The microlens may divertlight away from inner sub-pixel 202 towards outer sub-pixel group 204.

Toroidal microlenses 206 may be designed to steer most of the incominglight towards outer sub-pixel group 204 (e.g., to ensure a sufficientratio of the light sensitivity of the outer sub-pixel group to the lightsensitivity of the inner sub-pixel group). Illustrative toroidalmicrolenses 206 are shown in detail in FIG. 6. As shown in FIG. 6, eachtoroidal microlens 206 has an outer perimeter 212 and an inner perimeter214. The inner perimeter 214 defines an opening 216 in the toroidalmicrolens. As previously discussed, opening 216 in toroidal microlens206 may cover some or all of underlying inner sub-pixel 202.

In the embodiment of FIG. 6, toroidal microlenses 206 have a circularouter perimeter 212 and a circular inner perimeter 214. Additionally,each outer perimeter 212 fits within the boundary for the associatedpixel 200 (indicated by the dashed lines). In other words, themicrolenses are not merged together in FIG. 6. This example is merelyillustrative. In practice, the microlenses may be merged together asshown in FIG. 7.

In FIG. 7, each toroidal microlens again has a circular outer perimeter212 and circular inner perimeter 214. However, adjacent toroidalmicrolenses are merged together (e.g., at interface 218). This allowseach microlens to cover a greater area of its pixel 200. For simplicity,in subsequent descriptions herein the shape of outer perimeter 212 foreach microlens will be described without considering the degree ofmerging with adjacent microlenses. For example, in FIG. 7 outerperimeter 212 is considered circular (even though the circle merges withan adjacent microlens).

The merging of the microlenses as shown in FIG. 7 may occur at variousstages during manufacturing of the microlenses. In general, themicrolenses may be formed by depositing and patterning a photoresistmaterial (e.g., a photolithography process). After forming thephotoresist pattern, the photoresist material is heated to a temperatureat which the photoresist material softens and adopts a semi-sphericalshape (or other shape with curved outer surfaces). The photoresistmaterial may be only partially cured at this stage. Then, while thephotoresist material has the desired curved outer surfaces, a finalcuring process (e.g., added UV irradiation or heat) may maintain theshape of the photoresist material. Heating the photoresist material toimpart curvature may sometimes be referred to as a reflow process. Inone embodiment, half of the microlenses (e.g., in a checkerboardpattern) may first be completely formed (e.g., patterned, heated, andcured). Each of these microlenses may have a circular outer perimeter,for example. Then, once the first half of the microlenses are complete,the second half of the microlenses may be formed in between the firsthalf of the microlenses. The second half of the microlenses may contactand cover portions of the first half of the microlenses, forming mergedinterfaces 218 (e.g., merged interfaces 218 are formed after reflow andfinal curing of the first half of the microlenses). In an alternateembodiment, all of the microlenses may be formed in one process and adry etch transfer process (for example with gases CF₄ and C₄F₈) may beused to reduce the gaps and create the shapes shown in the figures. Inthis type of embodiment, merged interfaces 218 are formed by thephotoresist material of the microlenses merging during etch. Forsimplicity, in subsequent descriptions herein merged interfaces ofeither type may be considered a merged interface 218 between adjacentmicrolenses.

The example of FIGS. 6 and 7 of toroidal microlenses 206 having circularouter perimeters and circular inner perimeters is also merelyillustrative. In general, toroidal microlenses may have any desiredouter perimeter shape and inner perimeter shape. For example, to reducecross-talk at high incident light angles, microlens arrangements of thetype shown in FIGS. 8-11 may be used. To reduce cross-talk at highincident light angles, the microlenses may steer light towards thecorners of the pixels.

FIG. 8 is a top view of illustrative toroidal microlenses having planarouter perimeter portions. As shown in FIG. 8, microlenses 206 aresquared on four sides. In other words, instead of outer perimeter 212being circular, outer perimeter 212 has planar portions 220. Eachmicrolens 206 has four planar portions 220. Curved portions (e.g.,curved portion 221) of outer perimeter 212 are interposed between eachrespective pair of planar portions 220 of outer perimeter 212.Additionally, as shown in FIG. 8, inner perimeter 214 has a square shapeinstead of a circular shape. Inner perimeter 214 may have other desiredshapes if desired (e.g., non-square rectangular). Squaring the sides ofthe microlenses may provide better light separation at high incidentlight angles compared to the microlenses of FIG. 7.

In FIG. 8, each planar portion 220 has a respective width 222. The widthof the planar portion 220 at the outer perimeter 212 may be the same asor different from the width of the planar portion of inner perimeter214. Each microlens may have an overall width 224. Width 222 may be anydesired distance (e.g., less than 1 micron, less than 2 micron, lessthan 0.5 micron, less than 0.3 micron, less than 0.1 micron, greaterthan 1 micron, greater than 2 micron, greater than 0.5 micron, greaterthan 0.3 micron, greater than 0.1 micron, between 0.1 micron and 1.0micron, between 0.2 and 0.5 micron, etc.). Width 222 may be any desiredpercentage of width 224 (e.g., less than 40%, less than 20%, less than10%, less than 80%, less than 60%, less than 50%, greater than 40%,greater than 20%, greater than 10%, greater than 80%, greater than 60%,greater than 50%, between 10% and 30%, between 10% and 50%, between 10%and 90%, between 30% and 60%, between 25% and 75%, between 5% and 20%,etc.). The width shown in FIG. 8 is merely illustrative. If desired, alarger width for planar portions 220 may be used, as shown in FIG. 9.

Microlenses 206 in FIGS. 8 and 9 may still be described as toroidalmicrolenses (even though outer perimeter 212 is not a perfect circle).Microlenses 206 may be referred to as toroidal microlenses 206,ring-shaped microlenses 206, donut-shaped microlenses 206, or annularmicrolenses 206.

FIG. 10 shows another possible arrangement for microlenses over a pixelsuch as pixel 200 in FIG. 5 (with an inner sub-pixel group surrounded byan outer sub-pixel group). In FIG. 10, each pixel 200 is covered by morethan one microlens. The microlenses covering a given pixel may bereferred to as microlens group. Each microlens group includes a toroidalmicrolens 206 with an outer perimeter 212 and an inner perimeter 214.The inner perimeter 214 defines an opening 216 in the toroidalmicrolens. As previously discussed, opening 216 in toroidal microlens206 may cover some or all of underlying inner sub-pixel 202. In theembodiment of FIG. 10, toroidal microlenses 206 have a circular outerperimeter 212 and a circular inner perimeter 214 (as in FIGS. 6 and 7).However, this example is merely illustrative, and microlenses 206 mayhave outer and/or inner perimeters with planar portions (as in FIGS. 8and 9) if desired.

In addition to a toroidal microlens 206, each microlens group covering agiven pixel in FIG. 10 includes four additional microlenses (226-1,226-2, 226-3, and 226-4). A first microlens 226-1 is formed in theupper-left corner of the pixel over toroidal microlens 206, a secondmicrolens 226-2 is formed in the lower-left corner of the pixel overtoroidal microlens 206, a third microlens 226-3 is formed in theupper-right corner of the pixel over toroidal microlens 206, and afourth microlens 226-4 is formed in the lower-right corner of the pixelover toroidal microlens 206. Microlenses 226-1, 226-2, 226-3, and 226-4may be spherically shaped (e.g., may have a curved upper surface withoutan opening). Additional microlenses 226-1, 226-2, 226-3, and 226-4 mayfocus light on the corners of the pixels and reduce cross-talk. Thearrangement of FIG. 10 of each of additional microlenses 226-1, 226-2,226-3, and 226-4 being formed over a respective corner of toroidalmicrolens 206 is merely illustrative. In general, each microlens pixelgroup may include any desired number of additional microlenses (e.g.,one, two, three, four, more than four, etc.) in any desired arrangement.Each additional microlens may have any desired shape.

The additional microlenses may be formed in any desired manner. Aspreviously discussed, toroidal microlenses 206 may be formed in twoseparate photolithography processes (e.g., a first half of the toroidalmicrolenses are formed then a second half of the microlenses areformed). Additional microlenses 226-1, 226-2, 226-3, and 226-4 may thenbe formed over toroidal microlenses in a single photolithography processor multiple photolithography processes. Additional microlenses 226-1,226-2, 226-3, and 226-4 may be patterned directly on toroidalmicrolenses 206 such that the addition microlenses are in direct contactwith the toroidal microlenses.

FIG. 11 shows yet another possible arrangement for microlenses over apixel such as pixel 200 in FIG. 5 (with an inner sub-pixel groupsurrounded by an outer sub-pixel group). As shown in FIG. 11, a toroidalmicrolens group may be formed from individual microlenses that do nothave openings. Each pixel may be covered by four respective microlenses228-1, 228-2, 228-3, and 228-4. The four respective microlenses mayapproximate the shape of a toroidal microlens 206 with a central opening216 defined by the space between the four microlenses.

A first microlens 228-1 is formed in the upper-left corner of the pixel,a second microlens 228-2 is formed in the lower-left corner of thepixel, a third microlens 228-3 is formed in the upper-right corner ofthe pixel, and a fourth microlens 228-4 is formed in the lower-rightcorner of the pixel. Microlenses 228-1, 228-2, 228-3, and 228-4 may bespherically shaped (e.g., may have a curved upper surface without anopening). Microlenses 228-1, 228-2, 228-3, and 228-4 may focus light onthe corners of the pixels and reduce cross-talk. The arrangement of FIG.11 of each of microlenses 228-1, 228-2, 228-3, and 228-4 being formedover a respective corner of the pixel is merely illustrative. Ingeneral, each microlens group may include any desired number ofmicrolenses (e.g., one, two, three, four, more than four, etc.) in anydesired arrangement. Each microlens may have any desired shape.

Microlenses 228-1, 228-2, 228-3, and 228-4 may be shifted away from thecenter of their corresponding pixel 200. As shown, each pixel 200 has acenter (e.g., a geometric center) 230 within the opening 216 between themicrolenses. Each microlens has a respective center (e.g., geometriccenter) 232 that is separated from center 230 by a distance 234. Eachmicrolens center 232 may also be separated from a point 238 betweenadjacent microlens groups (e.g., center 230 is the center of fouradjacent microlenses within a single microlens group and point 238 isthe center of four adjacent microlenses in different microlens groups).Because each microlens is shifted away from the center of the pixel,distance 234 is greater than distance 236.

In general, the microlenses 228-1, 228-2, 228-3, and 228-4 may be formedby depositing and patterning a photoresist material (e.g., aphotolithography process). Then, reflow and final curing maintain thedesired shape of the curved outer surfaces of the microlenses. In oneembodiment, half of microlenses 228-1, 228-2, 228-3, and 228-4 (e.g., ina checkerboard pattern) may first be completely formed (e.g., patterned,heated, and cured). Each of these microlenses may have a circularperimeter, for example. Then, once the first half of the microlenses arecomplete, the second half of the microlenses may be formed in betweenthe first half of the microlenses. The second half of the microlensesmay contact and cover portions of the first half of the microlenses,forming merged interfaces (e.g., merged interfaces formed after reflowand final curing of the first half of the microlenses). In an alternateembodiment, all of microlenses 228-1, 228-2, 228-3, and 228-4 may beformed in one process and a dry etch transfer process (for example withgases CF₄ and C₄F₈) may be used to reduce the gaps and create the shapesshown in the figures. In this type of embodiment, merged interfacesbetween the microlenses are formed by the photoresist material of themicrolenses merging during etch.

In the aforementioned embodiments of FIGS. 6-11, each microlens may betuned based on the color of the pixel if desired. Each pixel has acorresponding color filter element (e.g., color filter element 104 inFIGS. 2A-2C) of a given color. The microlens group over the color filterelement of the given color may be tuned based on the given color (e.g.,based on the wavelengths of light transmitted by the color filterelement). For example, the microlenses may be formed with differentshapes, materials, refractive indices, positions, etc. based on thecolor of the pixel. Specifically, the location (e.g., shift relative tothe microlens) and size (e.g., diameter or width) of opening 216 may beselected based on the color of the pixel. The radius of curvature ofeach microlens may be selected based on the color of the pixel.Asymmetric microlens shapes or asymmetric positioning of the microlenseswithin a given pixel may be used for the microlenses if desired.

Additionally, in the aforementioned embodiments of FIGS. 6-11, opening216 in the center of the pixel (e.g., over inner sub-pixel group 202 inFIG. 5) may be left open or may be covered by an additional microlens.Opening 216 may have any desired shape and size.

In various embodiments, an image sensor may include an array of imagingpixels and each imaging pixel may include at least one photosensitivearea, a color filter element formed over the at least one photosensitivearea, and a toroidal microlens formed over the color filter element. Thetoroidal microlens may have an outer perimeter with a planar portion.

The planar portion may be a first planar portion and the outer perimetermay have a second planar portion, a third planar portion, and a fourthplanar portion. The outer perimeter may have a first curved portion, asecond curved portion, a third curved portion, and a fourth curvedportion. The first curved portion may be interposed between the firstand second planar portions, the second curved portion may be interposedbetween the second and third planar portions, the third curved portionmay be interposed between the third and fourth planar portions, and thefourth curved portion may be interposed between the fourth and firstplanar portions.

The toroidal microlens may have an inner perimeter that defines anopening in the toroidal microlens. The opening may be a square opening.The image sensor may also include an additional microlens formed overthe opening. The at least one photosensitive area may include an innerphotosensitive area and an outer photosensitive area that surrounds theinner photosensitive area. The opening may overlap the innerphotosensitive area. The toroidal microlens may have a first width, theplanar portion may have a second width, and the second width may bebetween 10% and 50% of the first width.

In various embodiments, an image sensor may include an array of imagingpixels and each imaging pixel may include at least one photosensitivearea, a color filter element formed over the at least one photosensitivearea, a toroidal microlens formed over the color filter element, and atleast one additional microlens formed on the toroidal microlens.

The at least one additional microlens may include first, second, third,and fourth additional microlenses. The first additional microlens may beformed on the toroidal microlens over a first corner of the at leastphotosensitive area, the second additional microlens may be formed onthe toroidal microlens over a second corner of the at leastphotosensitive area, the third additional microlens may be formed on thetoroidal microlens over a third corner of the at least photosensitivearea, and the fourth additional microlens may be formed on the toroidalmicrolens over a fourth corner of the at least photosensitive area. Thefirst, second, third, and fourth additional microlenses may be in directcontact with an upper surface of the toroidal microlens. The at leastone photosensitive area may include an inner photosensitive area and anouter photosensitive area that surrounds the inner photosensitive area.

In various embodiments, an image sensor may include an array of imagingpixels and an imaging pixel in the array of imaging pixels may includean inner sub-pixel that includes a first photosensitive area, an outersub-pixel that includes a second photosensitive area, a color filterelement formed over the inner sub-pixel and the outer sub-pixel, andfirst, second, third, and fourth microlenses formed over the colorfilter element. An opening formed from space between the first, second,third, and fourth microlenses may overlap the inner sub-pixel.

The first microlens may be formed over a first corner of the imagingpixel, the second microlens may be formed over a second corner of theimaging pixel, the third microlens may be formed over a third corner ofthe imaging pixel, and the fourth microlens may be formed over a fourthcorner of the imaging pixel. The first, second, third, and fourthmicrolenses may be in a 2×2 arrangement. The first, second, third, andfourth microlenses may each be shifted away from a center of the innersub-pixel. The outer sub-pixel may include the second photosensitivearea and a third photosensitive area, the second photosensitive area maybe overlapped by the first and second microlenses, and the thirdphotosensitive area may be overlapped by the third and fourthmicrolenses.

The foregoing is merely illustrative of the principles of this inventionand various modifications can be made by those skilled in the artwithout departing from the scope and spirit of the invention.

What is claimed is:
 1. An image sensor comprising an array of imagingpixels, wherein each imaging pixel comprises: at least onephotosensitive area; a color filter element formed over the at least onephotosensitive area; and a toroidal microlens formed over the colorfilter element, wherein the toroidal microlens has an outer perimeterwith a planar portion.
 2. The image sensor defined in claim 1, whereinthe planar portion is a first planar portion and wherein the outerperimeter has a second planar portion, a third planar portion, and afourth planar portion.
 3. The image sensor defined in claim 2, whereinthe outer perimeter has a first curved portion, a second curved portion,a third curved portion, and a fourth curved portion.
 4. The image sensordefined in claim 3, wherein the first curved portion is interposedbetween the first and second planar portions, wherein the second curvedportion is interposed between the second and third planar portions,wherein the third curved portion is interposed between the third andfourth planar portions, and wherein the fourth curved portion isinterposed between the fourth and first planar portions.
 5. The imagesensor defined in claim 1, wherein the toroidal microlens has an innerperimeter that defines an opening in the toroidal microlens.
 6. Theimage sensor defined in claim 5, wherein the opening is a squareopening.
 7. The image sensor defined in claim 5, further comprising: anadditional microlens formed over the opening.
 8. The image sensordefined in claim 5, wherein the at least one photosensitive areacomprises an inner photosensitive area and an outer photosensitive areathat surrounds the inner photosensitive area.
 9. The image sensordefined in claim 8, wherein the opening overlaps the innerphotosensitive area.
 10. The image sensor defined in claim 1, whereinthe toroidal microlens has a first width, wherein the planar portion hasa second width, and wherein the second width is between 10% and 50% ofthe first width.
 11. An image sensor comprising an array of imagingpixels, wherein each imaging pixel comprises: at least onephotosensitive area; a color filter element formed over the at least onephotosensitive area; a toroidal microlens formed over the color filterelement; and at least one additional microlens formed on the toroidalmicrolens.
 12. The image sensor defined in claim 11, wherein the atleast one additional microlens comprises first, second, third, andfourth additional microlenses.
 13. The image sensor defined in claim 12,wherein the first additional microlens is formed on the toroidalmicrolens over a first corner of the at least photosensitive area,wherein the second additional microlens is formed on the toroidalmicrolens over a second corner of the at least photosensitive area,wherein the third additional microlens is formed on the toroidalmicrolens over a third corner of the at least photosensitive area, andwherein the fourth additional microlens is formed on the toroidalmicrolens over a fourth corner of the at least photosensitive area. 14.The image sensor defined in claim 12, wherein the first, second, third,and fourth additional microlenses are in direct contact with an uppersurface of the toroidal microlens.
 15. The image sensor defined in claim11, wherein the at least one photosensitive area comprises an innerphotosensitive area and an outer photosensitive area that surrounds theinner photosensitive area.
 16. An image sensor comprising an array ofimaging pixels, wherein an imaging pixel in the array of imaging pixelscomprises: an inner sub-pixel that includes a first photosensitive area;an outer sub-pixel that includes a second photosensitive area; a colorfilter element formed over the inner sub-pixel and the outer sub-pixel;and first, second, third, and fourth microlenses formed over the colorfilter element, wherein an opening formed from space between the first,second, third, and fourth microlenses overlaps the inner sub-pixel. 17.The image sensor defined in claim 16, wherein the first microlens isformed over a first corner of the imaging pixel, wherein the secondmicrolens is formed over a second corner of the imaging pixel, whereinthe third microlens is formed over a third corner of the imaging pixel,and wherein the fourth microlens is formed over a fourth corner of theimaging pixel.
 18. The image sensor defined in claim 16, wherein thefirst, second, third, and fourth microlenses are in a 2×2 arrangement.19. The image sensor defined in claim 16, wherein the first, second,third, and fourth microlenses are each shifted away from a center of theinner sub-pixel.
 20. The image sensor defined in claim 16, wherein theouter sub-pixel includes the second photosensitive area and a thirdphotosensitive area, wherein the second photosensitive area isoverlapped by the first and second microlenses, and wherein the thirdphotosensitive area is overlapped by the third and fourth microlenses.